The present invention relates to inverter circuits and especially to inverter commutation circuits for use with inverters having semiconductors that are periodically triggered into conduction and periodically commutated off by applying voltage of reversed polarity to the dc input busses of the inverter. A problem in circuits of the prior art has been to provide sufficient voltage from the commutation capacitor at the start of commutation to commutate the inverter reliably, and especially when the dc voltage of the input busses of the inverter is adjustable to relatively low voltage values.
Some inverters of the prior art have been commutated off by charging a capacitor to a first polarity of voltage, then connecting the capacitor to dc input busses of the inverter with reverse polarity by means of semiconductor switches. In such inverters the reverse voltage applied to commutate the inverter is, at the start of commutation, the voltage to which the capacitor was initially charged. Other inverters have charged a commutation capacitor to a first voltage, then by means of switching and inductive-capacitive resonance, have discharged and recharged the capacitor to a reverse polarity, and applied the reverse polarity to the dc input busses of the inverter to commutate off the inverter semiconductors. The reverse voltage applied to the dc busses has risen gradually to a maximum reverse voltage because of the nature of the commutation circuits. Other commutation circuits of the prior art involve short-circuiting the dc input busses of the inverter before a reverse polarity of voltage is available for application to the dc bus to commutate off the inverter.
Other commutation circuits of the prior art are unsuitable for commutating inverters whose dc input busses receive variable voltage, which is adjustable down to relatively low voltage values; their commutation characteristics are unreliable at low bus voltages.
Some commutation circuits of the prior art have employed a separate dc source of voltage for charging a commutation capacitor prior to commutation, but either they have not subsequently reversed the polarity of the capacitor through a resonant circuit path other than the inverter itself, or else they have not delayed application of reverse voltage to the dc input busses of the inverter until a maximum reverse voltage has built up on the capacitor. In some prior circuits the separate dc source has provided a large pre-charge on the capacitor even at high output frequencies of the inverter.